The present invention relates to optical encoders, and in particular encoders using a hologram or other grating to read a moving grating, such as a CD.
Many conventional optical position encoder devices (also referred to as position sensors) illuminate the moving grating thereof using a incoherent light source which includes numerous independent point sources of different wavelengths. U.S. Pat. No. 4,879,510, U.S. Pat. No. 4,472,629 and U.S. Pat. No. 4,691,101, each of which is hereby incorporated in full by reference for all purposes, are examples of such optical position encoder devices.
FIG. 1(a) illustrates the basic principles of a conventional optical position encoder device 100. In conventional optical position encoder 100, an incoherent light source 101 illuminates a moving binary grating 102 (with the direction of movement indicated by the arrows). Moreover, a fixed grating 103 with the same period as moving binary grating 102 is placed adjacent to the moving binary grating. As the moving binary grating 102 moves across (e.g., along or parallel to) fixed grating 103, the amount of light falling on detector 104 (depicted by a dashed line in FIG. 1(a)) is dependent on the alignments of gratings 102 and 103. The largest amount of light is detected when gratings 102 and 103 are aligned in phase and the least amount of light is detected when grating 102 and 103 are aligned out of phase. The contrast of the modulated light is dependent on the adjacency (i.e., separation) of gratings 102 and 103. If the two grating are apart by more than a few periods of the gratings, detector 104 can not be used to sense the position of moving binary grating 102.
In order to increase the distance between gratings 102 and 103 beyond a few periods of the gratings, a conventional optical position encoder device configuration that includes a lens is used. Such a configuration is shown in FIG. 1(b). In FIG. 1(b), optical position encoder device 110 employs a lens 107 between a moving grating 106 and a fixed grating 108. Lens 107 produces an image of moving grating 106 on fixed grating 108. Also illustrated in FIG. 1(b) are an incoherent light source 105 and a detector 109 of optical position encoder device 110.
FIG. 2 shows an optical position encoder device 200 that is similar to optical position encoder device 110. However, in optical position encoder device 200, light source 201 and detector 205 are on the same side of the moving grating 202. Again, a lens 203 images moving grating 202 to fixed grating 204 disposed in front of detector 205.
The resolution of the conventional optical position encoder devices depicted in FIGS. 1(b), 1(b) and 2 is rather low. At the same time, the output signal from such optical position encoder devices is binary and not sinusoidal. These drawbacks make such conventional optical devices unsuitable for use as a high-resolution optical position encoder device.